The removal of undesired accompanying metals from aqueous solutions and the separation of metals is particularly important in hydrometallurgical nonferrous metal production, eg. in the winning of copper, zinc, noble metals or other special metals. Solutions of desired metals are frequently obtained from ores by digestion or leaching with aqueous, usually acid systems. The interfering or accompanying metals have to be removed from these solutions and separated from one another. In addition, the work-up of metal-containing wastes or residues (eg. flue dusts or precipitation sludges from wastewater treatment) and the recycling of used metal products (eg. catalysts) nowadays play an ever more important role in the provision of aqueous solutions of desired metals. Regardless of the origin of the metal salt solutions, it is always necessary to remove interfering elements from these solutions of desired metals and to separate the metals into individual fractions so that pure metals can be isolated. Apart from improving the quality of the desired metals, recovery of metals and reducing contamination of landfill areas is sought in the waste and residue processing sector for economic and ecological reasons.
The solvent extraction of iron ions is known in the literature. Thus, DE-A 38 01 430 (1) describes the use of a mixture comprising a primary amine and an alkylphosphonic monoester such as mono-2-ethylhexyl 2-ethylhexylphosphonate for the removal of iron(III) ions from acid zinc salt solutions by solvent extraction.
Furthermore, JP-A 1985/077936 (2) discloses that aminomethylenephosphonic acid derivatives are suitable for the solvent extraction of uranium, antimony or bismuth and of indium.
U.S. Pat. No. 4,741,831 (3) relates to a process for separating metals such as iron, cobalt, copper, vanadium, cadmium, nickel, zinc, lead or aluminum from aqueous solutions using water-soluble polymeric complexing agents, for example polyethyleneiminephosphonates. The metal complex is subsequently separated off by dialysis or ultrafiltration by means of membranes.
WO-A 96/00309 (4) describes the solvent extraction of iron ions from aqueous solutions, particularly from solutions of zinc or copper, by means of the present hydrocarbon-soluble aminomethylenephosphonic acid derivatives.
In the reference Proc. Symp. Solvent Extr. (1995) 59-60 (5), Y. Baba, Y. Kawano and J. Shibata describe the separation of palladium from chloride solutions by means of di (2-ethylhexyl) aminomethylenephosphonic acid.
However, the above processes of the prior art still have disadvantages. They are mostly not efficient enough and are too uneconomical. In particular, the selectivity of the separation of the interfering metals from the desired metals and the loading capacity of the complexing agents used are still in need of improvement.